Processing apparatus

ABSTRACT

The present invention relates to a processing apparatus for growing and annealing a silicon dioxide insulator layer to obtain a mobile positive ion free silicon dioxide insulator layer. A mobile positive ion free silicon dioxide insulator layer is required in order to make a stable insulated gate field effect transistor. The processing apparatus comprises a non-oxidizing, high-melting-point platinum metal film coated quartz furnace tube, potential means for placing a positive potential upon the platinum metal film coating of the platinum metal film coated quartz furnace tube to repel mobile ions therefrom, heater means for heating the interior of the quartz furnace tube, and gas means for passing oxygen gas through the platinum metal film coated quartz furnace tube. A silicon wafer may be oxidized in said processing apparatus to form a relatively mobile positive ion free silicon dioxide insulator layer of an insulated gate field effect transistor upon the silicon wafer. The silicon dioxide insulator layer is relatively uncontaminated by mobile positive ions which exist to the outside of the platinum metal film coated quartz furnace tube. A silicon wafer which previously has been coated by a silicon dioxide insulator layer may be processed to remove mobile ions within the silicon dioxide insulator layer. Mobile positive ions are repelled by the positive potential applied to the platinum metal film away from the outside of the quartz furnace tube, and mobile positive ions of the silicon dioxide insulator layer within the platinum metal film coated quartz tube are removed by the flowing oxygen gas due to the low vapor pressure of the mobile positive ions.

United States Patent [191 Koepp et al.

[11] 3,823,685 [451 July 16,1974

1 1 PROCESSING APPARATUS [75] Inventors: Ronald L. K0ePP.Dayton; Stanley J. Dudkowski, Kettering, both of Ohio [73] Assignee: The National Cash Register Company, Dayton,'0h io [22] Filed: Oct. 23, 1973 [21] Appl. No.: 408,700

Related US. Application Data [60] Continuation of Ser. No. 169,545, Aug. 5, 1971 abandoned, which is a division of Ser. No. 886,185 Oct. 14, 1969, Pat. NO. 3,645,695.

[52] U.S.'Cl. 118/49, '13/35 [51] Int. Cl. C230 13/08 [5 8] Field of Search 118/48-49.5;

117/106-1072, 93.1, 93.1 CD, 93.1 GD; 13/35; 148/174, 175

[56] References Cited UNlTED STATES PATENTS 2,955,566 10/1960 Campbell et al 118/48 3,098,763 7/1963 Deal et al. ll8/49.5 3,131,098 4/1964 Krsek et a1... 148/175 3,139,363 6/1964 Baldrey..... 264/81X 3,243,174 3/1966 Sweet 1187495 X 3,380,852 4/1968 Goetzberger 106/107 A X 3,492,969 2/1970 Emeis 118/49 1 3,571,478 3/1971 Teagan 13/35 X 3,594,242 7/1971 Burd et al...-. ll8/49.1 X 3,610,202 10/1971 Sussmann.... 118/48 3,635,771 l/l972 Shaw 148/175 Primary Examiner-Morris Kaplan [571 xnsrnxcr The present invention relates to a processing apparatus for growing and annealing a silicon dioxide insulator layer to obtain a mobile positive ion free silicon dioxide insulator layer. A mobile positive ion free silicon dioxide insulator layer is required in order to make a stable insulated gate field effect transistor. The processing apparatus comprises a non-oxidizing, highmelting-point platinum metal film coated quartz furnace tube, potential means for placing a positive potential upon the platinum metal film coating of the platinum metal film coated quartz furnace tube to repel mobile ions therefrom, heater means for heating the interior of the quartz furnace tube, and gasmeans for passing oxygen gas through the platinum metal film coated quartz furnace tube. A silicon wafer may be oxidized in said processing apparatus to form a relatively mobile positive ion free silicon dioxide insulator layer of an insulated gate field effect transistor upon the silicon wafer. The silicon dioxideinsulator layer is relatively uncontaminated by mobile positive ions which exist to the outside of the platinum metal film coated quartz furnace tube. A silicon wafer which previously has been coated by a silicon dioxide insulator layer may be processed to remove mobile ions within the silicon dioxide insulator layer. Mobile positive ions are repelled by the positive potential applied to the platinum metal film away from the outside of the quartz furnace tube, and mobile positive ions of the silicon dioxide insulator layer within the platinum metalfilm coated quartz tube are removed by the flowing oxygen gas due to the low vapor pressure of the mobile positive ions.

4 Claims, 5 Drawing Figures 1 PROCESSING APPARATUS cROss REFERENCE To RELATED APPLICATIONS This application is a continuation of co-pending application Ser. No. 169,545 filed on Aug. 5, 1971 now abandoned, which prior application is a division of ap-.

plication Ser. No. 866,185 filed on Oct. 14, 1969, and

issued as U.S. Pat. No. 3,645,695 on Feb. 29, 1972, all

assigned to thesame assignee.

BACKGROUND OF THE INVENTION U.S. Pat. No. 3,380,852, issuedApr. 30, 1968, on the application of Adolf Goetzberger, discloses a method of forming a relatively uncontaminated silicon oxide layer upon a silicon wafer, comprising placing a silicon wafer .within a quartz furnace tube, placing a wire electrode in close spatial relation above the silicon wafer, applying a positive potential to the electrode with respect to the silicon wafer, and oxidizing the silicon potential around a silicon wafer, by setting up an elec:

tric field between a positively charged wire electrode and the negatively-charged wafer.

In the processing apparatus of the present invention, a potential difference is not set up between the metal film coating of a metal film coated quartz furnace tube and a silicon wafer, but the silicon wafer exists within a positive equipotential closed metal film. The silicon wafer is not subjected to an electric field during its oxidation or during its annealing, but both during oxidation and during annealing, positive ions that may exist to the outside of the metal film coated quartz furnace tube are repelled and'prevented from entering the metal film coated quartz furnace tube, as a result of a positive potential existing upon the metal film coating of the metal film coated quartz furnace tube with respect to ground. Therefore positive ions are preventedfrom diffusing through the metal coated quartz furnace tube and combining with a silicon dioxide insulator layer during its formation upon the silicon wafer. Positive ions are also prevented from diffusing through the metal film coated quartz furnace tube to combine with a silicon dioxide insulator layer during its annealing. During annealing of' the silicon dioxideinsulator layer,- positive ions emitted from the silicon dioxide insulator layer are absorbed in a flowing oxidation gas which is passed through the interior of the metal film coated quartz furnace tube.

The positively-charged wire electrode of Goetzberger repels some positive ions that exist on the outside of the area between thepOsitively-charged wire electrode and the negatively-charged silicon wafer, but

does not repel positive ions which get into said area. In fact, positive ions which get into said area are absorbed by the negatively-charged silicon wafer.

The processing apparatus Ofthe present invention hindersthe combination of positive ions which may enter within the metal-film-coated quartz furnace tube with a silicon dioxide insulator layer therein, due to the flushing of the metal-film-coated quartz furnace tube with oxygen gas.

SUMMARY OF THE INVENTION A processing furnace tube having its inner wall composed of a metal which will not melt between a temperature of approximately 600 Centigrade and a temperaof 600 Centigrade and a temperature of 1,200

Centigrade.

DESCRIPTION OF THE DRAWING FIG. 1 is a perspective view of the processing apparatus of. the present invention used as an annealing furnace. v

FIG. 2 is a cross-sectional view of an externally c oated'processing furnace tube. 1

- FIG. 3 is a cross-sectional view of an internally coated passivated processing furnace tube.

FIG. 4 isa side view, with'parts broken away, of the processing apparatus of the present invention used as FIG. 5 is a cross-sectional view of an MOS field effect transistor having a positive sodium ion free silicon dioxide insulator layer.

DESCRIPTION OF THE PREFERRED I EMBODIMENT FIG. 1 shows a processing apparatus for annealing a silicon dioxide insulator layer to deplete the concentration of positive ions therein. As shown'in FIG. 1, a I

0.5-centimeter-thick furnace tube 12 has evaporated upon its inner surface 13 a 700-Angstrom-thick nonoxidizing, high-melting-point metal film 14 to form an inner wall therein. The metal film 14 is preferably a platinum or rhodium film. However, a tantalum or titanium film, with an oxygen-impervious silicon nitride layer thereon, can be used. The metal film 14, even though being an evaporated amorphous film on the furnace tube 12, is a good barrier to the passage of mobile ions, such as mobile positive sodium ions, due to its crystal structure.

In accordance with the present invention, a metal film 14 has been found to be necessary to make a resistively heated furnace tube, such as a quartz, aluminum oxide, silicon carbide, or silicon nitride furnace tube, impervious to ions, such as positive sodium ions, at high temperature. A resistively-h'eated furnace tube 12 will l,l00 Centigrade, at which temperature it is porous to posireach a temperature of approximately tive sodium ions. That is, sodiumions are highly mobile in diffusing through the crystalline structure of the furnace-tube 12. I

A platinum film 14 should be as thick as 600 Angstroms to stop most of the mobile ions from getting therethrough. However, a thinner film would be fairly effective in stopping mobile ions. An evaporated platinum film 14 which is made thicker than 600 Angstroms 3 is increasingly effective in stopping mobile ions. -.An evaporated platinum'film l4'which is thicker than 6,000 Angstroms begins to-peel from the quartz furnace tube 12. Therefore, a platinum film 14 which is between 600 Angstromsand 6,000 Angstroms thick is nace tube 6 itself, it is preferable that a platinum film be on the inside of the quartzfurnace tube 6.

In FIG. 1, the platinum film 14, being on the inner surface of the furnace tube" 12, hinders the mobile positive sodium ions within the wall of the furnace tube itself from'diffu'sing into the interior of the furnace tube 15 at high temperature. The platinum-film 14 does not melt at 1,100? Centigrade and does not oxidize at l,l Centigrade in an oxidizing atmosphere. Gold, which also is.a metal, melts near l,063 Centigrade. Therefore, gold cannot be usedto coat a-quartz furnace tube 12 which is heated 'to l,l00 Centigrade-Silver also melts below-1,100? Centigrade. A copper film melts below l,-l00 Centigrade. Therefore acopper film may not be used -to form a non-porous barrier to the passage of mobile ionsinto the. interior of the quartz processing furnace tube 12 when it is used in a high temperature oxidation processing apparatus.

As shownin FIG. 3, a tantalum film 10 may be evaporated upon the inside surface of a quartz furnace tube 9. The tantalum film 10 oxidizes in an oxidizing atmosphere, at l,l00 Centigrade. However, a layer 11 impervious to oxygen at high temperature, such as a silicon nitride layer, is placed upon the exposed surface of the 700-Angstrom-thick tantalum film 10. The silicon nitride layer 11 prevents oxygenin the interior of the tantalum-coated quartz furnace tube9 from oxidizing the tantalum film 10. The tantalum film 10 on the inner surface of the quartz furnace tube 9 prevents mobile ions from entering the interior of the quartz furnace tube 9. j I v A metal film'is preferred foruse in the processing appa'ratus of the present invention which has no mobile ions, has a high melting point, and does not oxidize at a high temperature. A platinum'film is a'very good metal film which'has these properties. Its atomic crystalline structure is fine enough to stop mobile ions from passing through it at high temperature.

As shown in FIG. 1, the platinum film,14, having been evaporated, is anamorphous platinum film. The evaporation causes the platinum film, which is evaporated on the inside of a quartz tube, to be amorphous. However; the evaporated film 14, being semicrystallineuis impervious to mobile ions.

It is to be observed that a solidmetal furnace tube,

, under the radiant heating coil-BOLThe battery 18 app'lies a positive 500 volts potential, with respect to ground, to the 700-Angstrom-thickplatinum film 14. By applying a positive potential to the platinum film 14 on the furnace tube 12, one can further hinderthe passage of mobile positive sodium ions from the'outside of the tube 12' into the interior of the furnace tube 12. r u

A silicon wafer holder 22 is laid within the platinum coated quartz furnace tube 12. A silicon wafer 24, having a 1,200-Angstrom-thick silicon oxide insulator layer 26, is held by the silicon wafer holder 22 within the platinum coated quartz furnace tube 12. The resistance heating'coil 30 is placed around the outside of the platinum-coated quartz furnace tubev 12.

The platinum film 14, is held at a positive 500 volts withv respect toground. Heat from the radiant heating coil 30, which is driven by an A. C. power. source 34, raises the temperature of the silicon wafer 24 within the center of the platinum-coated.quartz'furnac'e tube to 600 Centigrade. A 100 cc/rninute flowing oxygen gas.

from an oxygen container is passed through 95- Centigrade water 32 within a container 33, and then through the platinum-coated quartzfumace tube 12 while it isbeing heated, for 60 minutes. In accordance with the present invention, a l4-fold reduction in the numberof mobile positive ions, including mobile posicoated quartz furnace tube.l 2, over what can be obtained using an uncoated quartz furnace tube. A threefold-reductionin 'the number of mobile positive ions,

nut. Radiant heat,'from the resistanceheating coil 42, l

such as a platinum furnace tube, can be used in place edge of the platinum. film 14 willreach a temperature of only about 100 Centigrade, since it is not directly including mobile positive sodium ions, is achieved with the use of a positively-charged platinum-coated quartz furnace tube 12, over what can be obtained with the use of an uncharged platinum-coated tube 12;

a FIG. 4 shows a processing apparatus for oxidizing semiconductor material in a mobile-positive-ion-free furnace tube 40. As shown in FIG. 4, a platinum-coated quartz furnace quartzfurnace tube 40 is positioned by a suitable means within a resistance heating coil 42. A 700- Angstrom-thick platinum film 44 is evaporated upon the inside surface 45 of the quartz furnace tube 40. Oxygengas is passed through the quartz furnace tube 40 from an oxygen container 48 at "the'rate of 300 cc/miwhich is driven by an A. C. power source 50, raises the temperature inside thequartz furnace tube 40 to 1,l00

Centigrade. A positive potential of 500 volts from a battery 60 isatta'ched to the platinumfilm 44 byme'ans of a lead 62, with respect to ground. The oxygen gas in the platinum-coated quartz furnace tube 40 grows a relatively mobile-positive-ion-free silicon oxide insula tor layer on a silicon wafer 72, which is placed on a siliconwafer holder 74 in the platinum-coatedquartz furnace tube 40. An exit port allows the oxygen gas to exit from the quartz furnace tube 40. An 11,100.- Angstrom'silicon oxide insulator layer. 70 is grown upon an n-type silicon wafer 72, having p-typ'e Source and drain regions diffusedtherein, by oxidizing it for minutes. It is found that the silicon oxide insulator layer'70 which is produced has a ten-fold reduction in the concentration of mobile positively-charged ions therein over a silicon dioxide insulator layer that is produced in a quartz furnace tube which does not have a platinum film thereon. A two-fold reduction in. the concentration of mobile positive ions is produced therein from a silicon dioxide insulator layer produced in a platinum coated quartz furnace tube 40 which does not have a positive potential applied to the platinum film 44.

FIG. 5 shows an MOS field effect transistor 100 having a mobile-positive-ion-free silicon dioxide insulator layer 70 therein. As shown in FIG. 5, the silicon dioxide layer 70 is selectively etched so as only to extend from the edge of the p-type source region 99 to the edge of the p-type drain region 92.

A small area of a partially processed silicon wafer 72 can then be fabricated into a completed metal-silicon oxide-silicon (MOS) field effect transistor 100, as shown in FIG. 5. A gate electrode 102, such as an aluminum gate electrode, is deposited by vacuum evaporation upon the silicon oxide layer 70 through an evaporaton mask. A source electrode 103 and a drain electrode 105 are attached to the p-type source region 99 and to the p-type drain region 92, also by vacuum deposition. A metal-silicon oxide-silicon (MOS) field effect transistor is thereby produced.

Due to the formation of the silicon oxide insulator layer 70 within a mobile-positive-sodium-ion-free environment, mobile positive sodium ions are not trapped within the silicon dioxide insulator layer 70 of the MOS field effect transistor 100. The mobile-positive-sodiumion-free silicon dioxide layer 70, therefore, aids in producing an MOS field effect transistor 100, which begins to conduct a source-drain current, at a small -3 volts threshold voltage from a battery 110. The amountof source-drain current from the battery 112 also does not appreciably drift for a given gate voltage, under the periodic operation of the MOS field effect transistor 100. That is, the processing apparatus of the present invention aids in producing a silicon dioxide insulator layer 70, upon a silicon wafer 72, in such a way as to retard the trapping of mobile positive sodium ions within the silicon dioxide insulator layer 70 of the MOS field effect transistor 100. The decreased drift in the sourcedrain current for a given gate voltage, in the MOS field effect transistor 100, is due to the near lack of mobile positive sodium ions in the silicon dioxide insulator layer 70. If sodium atoms could migrate within the silicon dioxide insulator layer 70, a greater negative threshold voltage than "3 volts would be required, the added gate voltage being necessary to make up for the charge concentration of positive sodium atoms in the silicon dioxide insulator layer 70.

What is claimed is:

1. Apparatus for the thermal treatment of semiconductor material comprising:

a furnace tube having inner and outer wall surfaces;

a thin non-oxidizing, high-melting-point film of material disposed on the inner wall surface of the tube, the film .of material being impervious to mobile positive ions and selected from the group consisting of platinum; rhodium; tantalum overcoated with silicon nitride; and titanium overcoated with silicon nitride;

a source of positive potential;

means connecting the film to the source; and

means for introducing gases into the interior of the furnace tube.

2. Apparatus for annealing and reducing the concentration of mobile positive ions in a silicon oxide insulator layer in a furnace tube to deplete the concentration of mobile positive ions, including positive sodium ions, comprising:

a furnace tube having inner and outer wall surfaces;

a'thin non-oxidizing, high-melting-point film of material disposed on the inner wall surface of the tube, the film of material being impervious to mobile positive ions and selected from the group consisting of platinum; rhodium; tantalum overcoated with silicon nitride; and titanium overcoated with silicon nitride;

means for causing a gaseous material to flow through the furnace tube to anneal the silicon oxide insulator layer and to reduce the concentration of mobile positive ions therein; and

heater means in close proximity to the furnace tube for raising the temperature within the interior of the furnace tube to an elevated temperature range sufficient to drive mobile positive ions out of the silicon dioxide insulator layer.

3. Apparatus for annealing a silicon oxide insulator layer to deplete the concentration of mobile positive ions, including mobile positive sodium ions, in the silicon oxide insulator layer, comprising:

a furnace tube having inner and outer wall surfaces;

a thin non-oxidizing, high-melting-point film of material disposed on the inner wall surface of the tube, the film of material being impervious to mobile positive ions and selected from the group consisting of platinum; rhodium; tantalum overcoated with silicon nitride; and titanium overcoated with silicon nitride;

means for'applying a positive potential to the film for further hindering mobile positive ions from entering the interior of the furnace tube from the wall of the tube;

means connected to the furnace tube for causing a gaseous material to flow through the tube to anneal the silicon oxide insulator layer and to deplete the concentration of mobile positive ions therein; and

heating means acting in conjunction with the furnace tube for raising the temperature within the interior of the furnace tube to an elevated temperature range sufficient to drive mobile positive ions out of the silicon dioxide insulator layer.

4. Apparatus for oxidizing silicon semiconductor material in a mobile-positive-ion-free environment to form a silicon dioxide insulator layer thereon which is substantially free of mobile positive ions, comprising:

a furnace tube having inner and outer wall surfaces;

and to flush the interior of the furnace tube of mobile positive ions; and

heater means in close proximity to the furnace tube for raising the temperature within the interior of the furnace tube to an elevated temperature range sufficient to increase the kinetic energy of any mobile positive ions in the furnace tube. 

2. Apparatus for annealing and reducing the concentration of mobile positive ions in a silicon oxide insulator layer in a furnace tube to deplete the concentration of mobile positive ions, including positive sodium ions, comprising: a furnace tube having inner and outer wall surfaces; a thin non-oxidizing, high-melting-point film of material disposed on the inner wall surfaCe of the tube, the film of material being impervious to mobile positive ions and selected from the group consisting of platinum; rhodium; tantalum overcoated with silicon nitride; and titanium overcoated with silicon nitride; means for causing a gaseous material to flow through the furnace tube to anneal the silicon oxide insulator layer and to reduce the concentration of mobile positive ions therein; and heater means in close proximity to the furnace tube for raising the temperature within the interior of the furnace tube to an elevated temperature range sufficient to drive mobile positive ions out of the silicon dioxide insulator layer.
 3. Apparatus for annealing a silicon oxide insulator layer to deplete the concentration of mobile positive ions, including mobile positive sodium ions, in the silicon oxide insulator layer, comprising: a furnace tube having inner and outer wall surfaces; a thin non-oxidizing, high-melting-point film of material disposed on the inner wall surface of the tube, the film of material being impervious to mobile positive ions and selected from the group consisting of platinum; rhodium; tantalum overcoated with silicon nitride; and titanium overcoated with silicon nitride; means for applying a positive potential to the film for further hindering mobile positive ions from entering the interior of the furnace tube from the wall of the tube; means connected to the furnace tube for causing a gaseous material to flow through the tube to anneal the silicon oxide insulator layer and to deplete the concentration of mobile positive ions therein; and heating means acting in conjunction with the furnace tube for raising the temperature within the interior of the furnace tube to an elevated temperature range sufficient to drive mobile positive ions out of the silicon dioxide insulator layer.
 4. Apparatus for oxidizing silicon semiconductor material in a mobile-positive-ion-free environment to form a silicon dioxide insulator layer thereon which is substantially free of mobile positive ions, comprising: a furnace tube having inner and outer wall surfaces; a thin non-oxidizing, high-melting-point film of material disposed on the inner wall surface of the tube, the film being impervious to mobile positive ions and selected from the group consisting of platinum; rhodium; tantalum overcoated with silicon nitride; and titanium overcoated with silicon nitride; means for applying a positive potential to the film for further hindering mobile positive ions from entering the interior of the furnace tube; means for introducing a gaseous material into the furnace tube to oxidize the semiconductor material and to flush the interior of the furnace tube of mobile positive ions; and heater means in close proximity to the furnace tube for raising the temperature within the interior of the furnace tube to an elevated temperature range sufficient to increase the kinetic energy of any mobile positive ions in the furnace tube. 